Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures), and are widely used in automobiles and the like as well as in industrial applications. A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. For instance, in a centripetal turbine the turbine housing defines an annular inlet passageway around the turbine wheel and a generally cylindrical axial outlet passageway extending from the turbine wheel. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The rotating compressor wheel compresses inlet air and delivers the compressed air to the intake manifold of the engine, thereby increasing engine power.
It is also well known to provide turbochargers with a bypass passageway between the exhaust inlet and the exhaust outlet portions of the turbine housing to enable control of the turbocharger boost pressure and/or shaft speed. A wastegate valve is located in the bypass passageway and is controlled to open the passageway when the pressure level of the boost air increases towards a pre-determined level, thus allowing some of the exhaust gas to by-pass the turbine wheel preventing the boost pressure from rising above said level. The wastegate valve is generally actuated by a pneumatic actuator operated by boost air pressure delivered by the compressor wheel.
The conventional pneumatic actuator comprises a spring-loaded diaphragm (or a spring loaded sliding seal) housed within a canister, often referred to as the wastegate actuator can. The actuator can is connected by an airline to the compressor outlet and is generally mounted on the compressor housing. Communication with the compressor outlet is via a secondary boost air outlet passage such as a port formed in the compressor housing. The diaphragm/sliding seal acts on a connecting rod which extends to the wastegate valve assembly which is mounted in the turbine housing. The spring bias is such that under low boost pressure conditions the wastegate valve remains closed. However, when the boost pressure (which is transmitted to the actuator can from the compressor outlet via the secondary boost air outlet and the airline) reaches a predetermined maximum the diaphragm is moved against the action of the spring and operates to open the wastegate valve (via the connecting rod) thereby allowing some exhaust gas to bypass the turbine wheel.
The actuator can is generally connected to the compressor outlet by a flexible hose. In many cases the connection is direct. However, it is also known to provide a bleed valve, also referred to as a command valve, in the airline which responds to appropriate control signals (for instance from the engine management system) to effectively vary the pressure transmitted to the wastegate actuator can by venting to atmosphere. This provides for additional control of the wastegate valve over and above the basic operation dependent upon the pre-determined spring bias. For instance, the command valve may be operated to effectively modify the pressure at which the wastegate valve will begin to open which may be desirable in certain engine operating conditions.
Wastegate actuator control is not the only situation in which it is necessary to extract boost air from the outlet of a compressor via a secondary boost air outlet passage. For instance, one conventional method of preventing compressor surge is to install a solenoid valve in an air line ported to the compressor outlet via a secondary boost air outlet passage to controllably vent boost air from the outlet under conditions at which the compressor is likely to surge. The vented air may typically be vented to atmosphere via a silencer or returned to the compressor inlet for re-circulation through the compressor.
Where a valve is used to control the boost air flow through the secondary boost air outlet passage, it is quite common to mount the valve at a location remote from the compressor. For instance, some valves are constructed largely from plastic and therefore must be mounted away from the turbocharger at a cool part of the engine. Inevitably, remote mounting of the valve (whatever its function) requires extra air lines and associated fittings. A solution to this problem is provided by Japanese patent application S62-35565 (laid-open patent application S63-205419 filed in February 1987). This discloses mounting a solenoid valve directly to the compressor housing in order to avoid extra air line connections between the valve and the compressor. The compressor housing is provided with an integral valve mount located on an outside surface of the housing and including a first air port in communication with the compressor outlet volute and a second air port in communication with the compressor air intake. The mount has means for attaching the solenoid valve and forming a leak-tight seal between the valve and the first and second air ports provided in the valve mount. In this particular instance, the solenoid valve is provided for surge prevention and thus operates to selectively control flow of boost air from the compressor outlet to the compressor inlet (via the first and second air ports respectively) to avoid surge. Exactly the same valve mounting arrangement has been used for mounting a wastegate actuator command valve, see for instance U.S. Pat. No. 6,205,784.
A problem with the known arrangements for extracting air from a compressor outlet via a secondary boost air outlet passage is that any oil, dirt etc present in the air is fed to the downstream component (such as a valve or wastegate actuator) which over time can compromise the operation of the component. For instance, valve ports can become at least partially clogged by the build-up of oil and particulate material present in the boost air. This can be particularly problematical where the compressor is part of a turbocharger fitted to a combustion engine with a closed crank case ventilation system (CCV). Closed cranked case ventilation is becoming increasingly prevalent in order to meet modern stringent exhaust emission regulations.
It is an object of the present invention to obviate or mitigate the above disadvantages.